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Technique for Determining the Untrimmed Value of an Isolated Resistor in an Integrated Circuit

IP.com Disclosure Number: IPCOM000042596D
Original Publication Date: 1984-Jun-01
Included in the Prior Art Database: 2005-Feb-04
Document File: 2 page(s) / 39K

Publishing Venue

IBM

Related People

Bartley, GK: AUTHOR [+4]

Abstract

Thick and thin film resistor technologies are widely used in electronic packaging schemes. One of the limitations in utilizing these resistor technologies is that the sheet resistance of the materials varies by as much as #50% from nominal. The standard technique for overcoming this drawback is to overdesign the basic resistor geometry and then trim the resistor back to the desired value with a laser. This "passive" method normally requires sensing the value of the resistor as the trim is done. Sensing the resistor value is usually accomplished by forcing the contacts of the resistor to be available at the package I/O pins. In a multichip module environment, all nets are not functionally available at module I/Os.

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Technique for Determining the Untrimmed Value of an Isolated Resistor in an Integrated Circuit

Thick and thin film resistor technologies are widely used in electronic packaging schemes. One of the limitations in utilizing these resistor technologies is that the sheet resistance of the materials varies by as much as #50% from nominal. The standard technique for overcoming this drawback is to overdesign the basic resistor geometry and then trim the resistor back to the desired value with a laser. This "passive" method normally requires sensing the value of the resistor as the trim is done. Sensing the resistor value is usually accomplished by forcing the contacts of the resistor to be available at the package I/O pins. In a multichip module environment, all nets are not functionally available at module I/Os. This problem can be overcome by temporarily sharing I/Os with resistors whose contacts do naturally appear at the I/Os. By pairing resistors whose contacts naturally occur at I/Os with those that do not, and making sure that they terminate at the same voltage, a simple connection can be made which allows the two reistors to temporarily share the logical I/O, as illustrated in Fig. 1. Sensing the resistance REQ between pin S1 [I/O] and pin V [I/O] yields REQ = R1//R2 = R1R2. R1+R2 Before the laser begins to trim R1, the temporary connection to R2 is severed; now sensing the resistance between S1 [I/O] and V [I/O] yields R1. Thus, R2 = R1 R2, R1 - REQ and a precise determination of the untrimmed value of R2 has been made. Fig. 2 shows a physical cell design of two resistors according to Fig. 1, including the temporary connection between S1 and S2. This connection is automatically severed by one of the laser kerfs through R1 when the trim of R1 is started. The remaining sho...